This invention relates to processes for the production of thyroglobulin. Thyroglobulin, a glycoprotein produced by the thyroid gland, functions as a source of the active hormones levothyroxine (C.sub.15 H.sub.11 I.sub.4 NO.sub.4, also referred to as thyroxine) and triiodothyronine (C.sub.15 H.sub.12 I.sub.3 NO.sub.4, also known as liothyronine and thyronine). These hormones are formed from the iodinated tryosine moieties of thyroglobulin and are released into blood by proteolysis of thyroglobulin.
Thyroglobulin is utilized in the treatment of hypothyroidism, e.g., as an orally administered thyroid supplement. Specific diseases which can be treated with thyroglobulin include cretinism, myxedema, and ordinary, primary, secondary, and/or tertiary hypothyroidism. The components, structure, physical properties, and biological activity of thyroglobulin are known in the art. The product is available is prescription form. See, for example, Physicians Desk Reference 1989, p. 1593, "Proloid".RTM. brand of thyroglobulin; S. Lissitzky, Pharmacol. Ther. B 2, 219 (1976); and G. Levy et al., Am. J. Pharm. 133, 255 (1961).
The source of commercially produced thyroglobulin is the thyroid glands of hogs. These glands are ground and digested in a saline solution. After separating the meat tissue, the resulting aqueous solution is further treated (e.g., precipitation, denaturing, defatting, and drying).
The thyroglobulin product obtained is a free-flowing powder of cream to tan color and having a slight, characteristic odor. Powdered thyroglobulin is used, for example, in the production of Proloid.RTM. tablets.
As mentioned above, thyroglobulin functions as a source of thyroxine and triiodothyronine. Thus, an important parameter associated with thyroglobulin is the ratio of thyroxine, i.e., T.sub.4, to triiodothyronine, i.e., T.sub.3, also known as the T.sub.4 /T.sub.3 ratio, as well as the absolute amounts of these active ingredients upon proteolysis.
The United States Pharmacopea (USP) has recently changed the USP specification for thyroglobulin and thyroglobulin tablets in order to provide the physician with a standard dosage range based on active T.sub.4 and T.sub.3 levels, since both synthetic and glandular products are now available to physicians. This new specification insures better dosimetry with the glandular-derived product, particularly when compared to the competitive and well-defined synthetic thyroid products.
The older USP XX specification for thyroglobulin required a minimum of 0.7% of organically bound iodine as determined by the potassium carbonate fusion technique (see USP XX, pp. 799-800). This specification and classical preparations, as well as that for the formulated tablets (p. 800), allowed for significant fluctuation of active ingredient (as available T.sub.4 and T.sub.3 constituents) when compared to synthetic thyroid preparations of known absolute active ingredient composition [see S. Ingbar et al., New Eng. J. Med. 270, 439 (1964)]. The most recent USP XXII specification on formulated product (p. 1372) specifies the ratio of T.sub.4 /T.sub.3 of 3 and requires that the tablets contain 85-115% of the labeled amount of levothyroxine and 90-110% of the labeled amount of liothyronine, which labeled amounts are equal to 36 .mu.g levothyroxine and 12 .mu.g of liothyronine for each 65 mg of the labeled content of glandular derived thyroglobulin.
At the same time, the specification for the glandular derived product was simply set at 90-110% of the labeled amounts of levothyroxine and liothyronine. These newer specifications require the production of glandular derived thyroglobulin with allowable T.sub.4 /T.sub.3 ratios of 2:4-3.6 and absolute levels not less than 0.7 .mu.g/mg of liothyronine and 2.1 .mu.g/mg levothyroxine as determined by the proteolytic enzyme/HPLC assay.
Although conventional processes for the production of thyroglobulin routinely met USP XX specifications, the new USP XXII specification could not consistently be met with existing processes since the old process routinely product with T.sub.4 /T.sub.3 ratios which could not be used to produce certain of the available dosage forms under the newer specification for dosage forms.
In prior in-house processes, the desired T.sub.4 /T.sub.3 ratio and absolute values of levothyroxine and liothyronine of product thyroglobulin which could subsequently be employed to prepare dosage forms meeting USP XXII specifications were only achieved in about 30% of the process runs. Thus, such processes were economically disadvantageous.